Method and device for inspecting defect of sheet-shaped transparent body

ABSTRACT

A method for detecting a flaw accompanied with an optical defect if any on or in a sheet-like transparent body which is moved, and determine its type comprises placing an illuminator on one side of a sheet-like transparent body, and placing a (one-dimensional) image pickup on the opposite side. The illuminator means comprises lighting and darkening portions, and the image pickup is placed relative to the illuminator such that the boundary between the portions appears on the image pickup as a straight line in parallel with the long axis of the pickup. Image data created from the image pickup, are subjected to contrast enhancement to provide contrast enhanced image data which are displayed as a contrast enhanced image. The sequence (flaw pattern) of light spots and dark spots appears in the enhanced image as of the sheet-like transparent body is moved, and determines the type of flaw.

TECHNICAL FIELD

The present invention relates to a testing method for detecting a flawaccompanied with an optical defect in a sheet-like transparent body suchas a transparent plate like a glass plate, or a transparent plate coatedwith a transparent film, and a testing system for detecting such a flaw.

BACKGROUND ART

The flaw which may be present in a transparent plate such as a glassplate includes indentations which may exist on the surface, foreignobjects falling from above, and resting on and adhering to the surface,a crater-like pit and edge left by such a falling foreign object, and aforeign object and air bubble entrapped in the glass body. If thetransparent plate is a glass plate whose surface is coated with atransparent film, the flaw may include a pinhole on the surface. If theflaw is accompanied with an optical defect, light incident upon thesurface damaged with the flaw is abnormally refracted. A glass platehaving any flaw accompanied with an optical defect must be identifiedvia test to be rejected because such a plate is not utilizable as anoptical substrate.

A testing method for detecting an optical defect of a glass plate isdisclosed in the Japanese Patent Publication No. 8-247954. The testingmethod described in this publication consists of changing the positionof a test object with respect to a light source and an image recordingdevice, allowing the test object to pass behind a boundary edge betweena light scattering portion and a light shielding portion, comparingimage data acquired before the object passes behind the boundary edgeand image data acquired after the object has passed behind the boundaryedge, and detecting an optical defect if any based on an absolute valueof the difference between the two data. However, according to thismethod of detecting an optical defect based on an absolute value of thedifference between the two image data, one acquired before the objectpasses behind the boundary edge and the other acquired after the objecthas passed behind the boundary edge, it will not be possible todistinguish different kinds of flaws accompanied with an optical defectfrom one another, e.g., between a foreign object and an air bubble bothentrapped in a glass plate.

Another method disclosed in the Japanese Patent No. 3178644 (U.S. Pat.No. 5691811A and EP 0726457A2) consists of allowing light from a lightsource to pass through a mesh structure placed close to the light sourcewhere plural thin light shielding septa and light transmitting slitsrepeat themselves alternately, and to impinge on a test transparentplate and penetrate the test plate, and allowing then a linearlyextended camera placed opposite to the light source with the testtransparent plate in between to receive light carrying an imageconsisting of a stripe of dark and light bars, which then transmits theimage to an image processing unit so that a flaw if any in the testtransparent plate can be detected through the analysis of the image.According to this method, detection of a flaw accompanied with anoptical defect as distinct from a simple blemish such as a dust or soiladhered on the surface is achieved by displacing the focus of the cameraapart from the mesh structure such that the black (dark) barscorresponding with the septa and white (light) bars corresponding withthe slits constituting the stripe overlap with each other to give a greystrip, that is, displacing the focus of the camera to a first positionwhere maximally bright bars and minimally bright bars merge ascompletely as possible to give a homogeneous bright strip where thedifference in brightness between the maximally bright bands andminimally bright bands is minimized, which is called a flaw detectionposition, and keeping the focus at that position. Actual detection of aflaw accompanied with an optical defect consists of checking for thepresence of a flag-like image signal.

Although this method allows one to distinguish a flaw accompanied withan optical defect from a simple blemish devoid of optical defect such asa dust or soil, it is impossible by this method to distinguish twodifferent kinds of flaws both accompanied with an optical defect, e.g.,to distinguish between a foreign object and an air bubble both entrappedin a test object. The method may include the use of a linearly extendedmesh structure, crosswise combined mesh structures obtained by combiningtwo linearly extended mesh structures extending in two directionscrossing with each other at right angles, or a checker board patternedmesh structure obtained by arranging light shielding septa and lighttransmitting slits alternately in rows into a checkerboard pattern. Inno matter what pattern the mesh structures may be arranged, one can notdistinguish two different kinds of flaws both accompanied with anoptical defect in a test object solely dependent on the pattern of lightand dark spots obtained from light penetrating the structures: the sameflaws may cause different patterns of light and dark spots depending,e.g., on the width of the septa or slits, or on refractions of lightincident on them, or two different kinds of flaws may cause the samepattern of light and dark spots. Here, it is appropriate to assume, asan illustration, that there are, in a test transparent plate, a foreignobject extending in a direction in which the plate is moved, a secondforeign object and an air bubble. The first foreign object may cause apattern comprising “light, dark, light, dark and light” spots, thesecond foreign object may cause a pattern comprising “dark, light, darkand light” spots, and the air bubble may cause a pattern comprising“dark, light, dark and light” spots. Thus, one cannot distinguish thethree flaws from each other simply dependent on the pattern of dark andlight spots carried by light passing through the mesh structure.

DISCLOSURE OF THE INVENTION

One object of the present invention is to provide a testing method bywhich one can detect a flaw accompanied with an optical defect if any ina sheet-like transparent body in such a manner as to identify the typeof the flaw, and a system suitable for executing the method.

Another object of the invention is to provide a testing system withwhich one can detect a flaw accompanied with an optical defect if any ina sheet-like transparent body as distinct from a dust not accompaniedwith any optical defect and simply adherent on the surface of thesheet-like transparent body.

A first aspect of the invention is a testing method for detecting a flawaccompanied with an optical defect if any in a sheet-like transparentbody which is carried from one place to another. According to this flawcheck method, an illuminating means is placed on one side of a testsheet-like transparent body, while a linearly extended (one-dimensional)image pickup means is placed on the opposite side. The illuminatingmeans consists of a lighting portion and darkening portion, and isplaced with respect to the image pickup means such that the boundarybetween the lighting and darkening portions is projected onto the imagepickup means as a straight line running essentially in parallel with thelatter means, and that the image of the boundary falls on the visualfield of the image pickup means. According to this method, atwo-dimensional image is reconstructed from one-dimensional outputsprovided by the image pickup means; a contrast-enhanced image isproduced by subjecting the two-dimensional image to three-value basedcontrast enhancement, and a flaw is detected and its type is determinedbased on the pattern of light and dark spots sequentially appearing inthe contrast-enhanced image with movement of the test sheet-liketransparent body.

A second aspect of the invention is a flaw check system for detecting aflaw accompanied with an optical defect if any in a sheet-liketransparent body which is carried from one place to another. The flawdetection system comprises a linearly extending (one-dimensional) imagepickup means placed on one side of a test sheet-like transparent bodyand an illuminating means placed opposite to the image pickup means withrespect to the test transparent body. The illuminating means consists ofa lighting portion and darkening portion, and is placed with respect tothe image pickup means such that the boundary between the lighting anddarkening portions is projected onto the image pickup means as astraight line running essentially in parallel with the latter means, andthat the image of the boundary falls on the visual field of the imagepickup means. The flaw detection system further comprises an imageprocessing device in which a two-dimensional image is reconstructed fromone-dimensional outputs provided by the image pickup means, acontrast-enhanced image is produced by subjecting the two-dimensionalimage to three-value based contrast enhancement, and a sequentialpattern (flaw pattern) of light spots and dark spots is traced whichappears in the contrast enhanced image with movement of the sheet-liketransparent body.

According to the testing method and system of the invention,identification of the type of a flaw is achieved by tracing in whatorder or in what pattern light spots and dark spots appear or combinewhen light impinging on a flaw is singularly deflected therewith while atest sheet-like transparent body carrying the flaw moving in onedirection, and it is possible thereby to distinguish at least two ormore flaws chosen from the group comprising an entrapped foreign object,entrapped air bubble, bump, notch, adhered foreign object, print left bya falling foreign object, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram for illustrating the basic composition ofa testing system embodying this invention.

FIG. 2 is a diagram to show how the illuminating means looks like whenthe system of FIG. 1 is viewed by human naked eyes from its line sensor.

FIGS. 3A and 3B are diagrams to show how the illuminating means looklike when viewed by human naked eyes.

FIGS. 4A, 4B and 4C illustrate how a flaw consisting of a bump on thesurface of a test glass plate is detected and identified.

FIGS. 5A, 5B and 5C illustrate how a flaw consisting of an air bubbleentrapped in a test glass plate is detected and identified.

FIG. 6 presents an image obtained from image data derived from a flawconsisting of an entrapped air bubble.

FIG. 7 is a contrast-enhanced image obtained by subjecting the imagedata derived from a flaw consisting of an entrapped air bubble tothree-value based contrast enhancement.

FIG. 8 presents an image obtained from image data derived from a flawconsisting of an entrapped foreign object.

FIG. 9 is a contrast-enhanced image obtained by subjecting the imagedata derived from a flaw consisting of an entrapped foreign object tothree-value based contrast enhancement.

FIG. 10 shows another testing system to which a second light shieldingplate is added.

FIG. 11 is a diagram to show how the illuminating means looks like whenthe system of FIG. 10 is viewed by human naked eyes from its linesensor.

FIGS. 12A, 12B, 12C and 12D illustrate how a flaw consisting of an airbubble entrapped in a test glass plate is detected by the testing systemshown in FIG. 10.

FIGS. 13A and 13B show the profiles of two flaws: one consists of afalling foreign object on the surface of a test glass plate, and theother of a crater-like flaw consisting of a central pit and surroundingedge left by such a falling foreign object.

FIGS. 14A and 14B show the patterns of “light spots” and “dark spots”representing a falling foreign object and a crater-like print left by afalling foreign object.

FIG. 15 shows the profile of a contrast-enhanced image obtained bysubjecting image data representing a dust resting on the surface of atest glass plate to three value based contrast enhancement.

FIG. 16 shows another illuminating means useful for the testing systemof the invention.

FIG. 17 shows yet another illuminating means useful for the testingsystem of the invention.

FIG. 18 shows yet another illuminating means useful for the testingsystem of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1

FIG. 1 shows the composition of a flaw detection system 1 representing afirst embodiment of the invention. The flaw detection system comprisesan illuminating means 2, an image pickup means 4, and an imageprocessing means 6. The illuminating means 2 comprises a light shieldingplate 22 which forms its darkening portion, and a light source 21 whichforms its lighting portion. The image pickup means 4 is a line sensor(resolution being 50 μm) consisting of a sensor beam scanned linearlyand repetitively, and comprises a line sensor camera 41 and a lens 42. Atest glass plate 3 is moved by a moving means 5 through a space betweenthe illuminating means 2 and image pickup means 4 configured asdescribed above. The direction of the movement is towards left whenviewed from front as indicated by the arrow of the figure.One-dimensional output signals from the line sensor 4 are transmitted tothe image processing means 6. Based on the one-dimensional outputsignals, the image processing means 6 produces image data, subjects theimage data to three value based contrast enhancement to enhance thecontrast of an image represented by the image data, and displays thethus contrast enhanced image comprising “light spots” and “dark spots”the sequential pattern of which indicates the presence of a flaw if anyand its type.

The light source 21 or lighting portion of the illuminating means 2 is astraight tubular fluorescent lamp. If the fluorescent lamp is lightedvia a power source commercially available, its light intensity will besubject to considerable fluctuations. To avoid this, a high frequency ACpower source is used for activating the fluorescent lamp. The drivingfrequency is set to 30 kHz or higher. Illumination due to a fluorescentlamp activated via a high frequency AC power source is desirable becauseit requires a low cost and is subject to little fluctuation.Particularly, when a linearly extended light source is required, astraight tubular fluorescent lamp is ideal.

Above the fluorescent lamp 21 is placed the light shielding plate 22 ora darkening portion of the illuminating means 2. The light shieldingplate 22 is placed with respect to the line sensor camera 41 such thatthe edge line of the light shielding plate corresponds with the opticalaxis 7 of line sensor 4, and that said edge line is placed downstream ofthe optical axis 7 in terms of the movement of a test glass plate. Thelight shielding plate 22 is preferably colored black, more preferablymatted black, so that its image on the line sensor or a dark strip issharply contrasted with an adjacent light strip or an image of the lightsource. To enhance the brightness of the light strip, it is preferableto shift the central axis of fluorescent lamp 21 towards right withrespect to the optical axis 7 of line sensor 4 as shown in FIG. 1.

FIG. 2 is a diagram to show how the illuminating means looks like whenthe system is viewed by human naked eyes from a position close to linesensor 4. The shade of light shielding plate 22 forms a dark strip whilelight from the lighting surface of fluorescent lamp 21 not interceptedby the light shielding plate 22 forms a light strip. The cross mark atthe boundary between the dark and light strips indicates the center ofthe visual field of line sensor. The line sensor operates by scanning asensor beam linearly along the boundary between the dark and lightstrips, and converting the distribution of light intensity along theboundary into corresponding signals.

Above the illuminating means is placed the moving means 5 for moving atest glass plate 3 in one direction. In the particular embodiment shownin FIG. 1, the moving means comprises a set of rollers which rotate soas to move the plate in the direction as indicated by the arrow in thefigure. Namely, the moving direction is perpendicular to the directionin which the boundary between the dark and light strips extends.

Above the moving means is placed the line sensor 4 comprising linesensor camera 41 and lens 42. The line sensor camera is placed withrespect to the illuminating means 2 such that the boundary between thedark and light strips falls within the visual field of the line sensorcamera as described above. The visual field of a line sensor has acertain length, and thus an appropriate number of line sensors may beintroduced in the flaw detection system depending on the width of aglass plate to be tested. The line sensor camera 42 is preferablyfocused close to a test transparent plate, preferably to the surface ofa test transparent plate.

The image processing means 6 may include, for embodiment, a computer. Ifoutput from the line sensor comprises analog signals, the imageprocessing means 6 should be further provided at least with an imagedata receiving unit capable of analog/digital (A/D) conversion, i.e.,ability to convert analog signals into digital signals, because thecomputer can handle only digital signals. On the contrary, if the linesensor camera is a digital camera, need for A/D conversion can be safelyavoided.

In the following will be described the principle underlying theoperation of the flaw detection system, i.e., how the system can detect,when it receives a glass plate 3 with a flaw for inspection, the flawand determine its type depending on the pattern of “light spots” and“dark spots” reflecting abnormal refractions of light due to the flaw(flaw pattern) which sequentially appear on the screen as the glassplate 3 moves through the system.

FIG. 1 gives diagrams to show how the illuminating means looks like onthe surface of a test glass plate when viewed by human naked eyes placedat the line sensor 4. FIG. 3A shows an image of the illuminating meanswhen the test glass plate has no flaw. The left-side black striprepresents the shade of light shielding plate 22 or the darkeningportion of the illuminating means, while the right-side white striprepresents an image of light source 21 or the lighting portion of theilluminating means. If a glass plate portion carrying a flaw consistingof a bump comes to the boundary between the dark and light strips, lightimpinging on the bump is refracted so abnormally on account of theconvex lens action of the bump that a light spot appears against thedark strip 22 and then a dark spot appears against the light strip 21 asindicated by numeral 10 in FIG. 3B. FIGS. 3A and 3B indicate how theilluminating means looks like when viewed by human naked eyes from aposition close to the line sensor. If the same image is viewed by theline sensor which is focused onto the surface of the same glass plate,the boundary between the light and dark strips becomes more blurred, andtakes a lightness intensity intermediate between those of the light anddark strips. To let the intermediate light intensity exactly fall at amid-point between those of the light and dark strips, it is necessary todetermine an output from the line sensor after removing the lightshielding plate 22, to take the output as 100, and to adjust theposition of light shielding plate, when introducing again the lightshielding plate into the system, such that an output corresponding with50 is obtained from the line sensor.

Because the glass plate is constantly moved, the line sensor firstlydetects a light spot 21 and then a dark spot 22 occurring as a result ofdefective refraction due to the bump 10 as described later. Outputsignals from the line sensor are converted by the image processing meansinto image data which are then subjected to three value based contrastenhancement to enhance the contrast of an image represented by the imagedata, and the thus contrast enhanced image comprising “light spots” and“dark spots” is displayed on the screen. It is determined by analyzingthe pattern of “light and dark” spots that the flaw includes at least abump.

A flaw pattern appearing in association with a bump has been described.Next, what pattern of “light and dark” spots appears in association witha notch will be described, together with a pattern appearing inassociation with an air bubble entrapped in a glass plate.

FIG. 4 illustrates how a flaw consisting of a bump on the surface of atest glass plate is detected and identified as described above withreference to FIGS. 3A and 3B. FIG. 4A illustrates how the line sensor 4receives light penetrating the front half of bump 31 formed on thesurface of glass plate 3 which is moved in one direction. The opticalaxis of the majority of light impinging on the line sensor 4 isdeflected towards the lighting portion of fluorescent lamp 21 from theedge of light shielding plate 22 as a result of the convex lens actionof bump 31. Therefore, at this moment, the flux of light impinging onthe line sensor becomes larger than that of light penetrating the normalglass plate portion having no such bump.

FIG. 4B illustrates how the line sensor 4 receives light penetrating therear half of bump 31 formed on the surface of the glass plate which ismoved. The optical axis of the majority of light impinging on the linesensor 4 is deflected from the edge of light shielding plate 22 towardsthe center of the same plate on account of the convex lens action ofbump 31. Therefore, at this moment, the flux of light impinging on theline sensor becomes smaller than that of light penetrating the normalglass plate portion having no such bump.

If the system inspects, using the line sensor, a test glass plate havinga bump on its surface as described above while the plate is moving,outputs I (representing the light intensity) from the pixels of the linesensor including those receiving light penetrating the bump will give atrace as shown in FIG. 4C. In FIG. 4C, the ordinate represents an outputper one pixel of the line sensor, and the abscissa the moving distance Lof the glass plate.

In FIG. 4C, the output I₀ represents an output per one pixel when theline sensor observes a blurred image of the boundary between the lightstrip 21 and dark strip 22 as shown in FIG. 3A. However, when an imageof a bump comes in the visual field of the line sensor, output from theline sensor becomes higher than the level I₀ to give a peak which isfollowed by a trough smaller than I₀.

Outputs from the line sensor are fed via an image data receiving unit toa computer where the outputs are processed to produce image data. Imagedata are then subjected to three value based contrast enhancement toenhance the contrast of an image represented by the image data, and thethus contrast enhanced image is displayed on the screen. In this case,in correspondence with a bump, there appear on the screen “light spots”and “dark spots” in this order, or a flaw pattern of “light and dark”profile.

Incidentally, it is found that a foreign object entrapped in a glassplate and accompanied with an optical defect causes a flaw pattern of“light and dark” profile as does a bump.

It will be readily appreciated that a notch formed on the surface of aglass plate will generate a contrast enhanced image where “dark spots”and “light spots” appear in this order as opposed to the patterngenerated by a bump. A pinhole formed on a glass plate whose surface iscoated with a thin film is found to generate a flaw pattern of “dark andlight” profile similarly to a notch.

Next, the pattern which will be generated by an air bubble entrapped ina glass plate will be described below. FIG. 5A illustrates how the linesensor 4 receives light penetrating the front half of an air bubble 32entrapped in a glass plate 3 which is moving. The optical axis of themajority of light impinging on the line sensor 4 is deflected from theedge of light shielding plate 22 towards the center of the same plate onaccount of the lens action of air bubble. Therefore, at this moment, theflux of light impinging on the line sensor becomes smaller than that oflight penetrating the normal glass plate portion having no such airbubble.

FIG. 5B illustrates how the line sensor 4 receives light penetrating therear half of air bubble 32 entrapped in the glass plate which is moving.The optical axis of the majority of light impinging on the line sensor 4is deflected towards the lighting portion of fluorescent lamp 21 fromthe edge of light shielding plate 22 as a result of the lens action ofair bubble. Therefore, at this moment, the flux of light impinging onthe line sensor becomes larger than that of light penetrating the normalglass plate portion having no such air bubble.

If the system inspects, using the line sensor, a test glass plateentrapping an air bubble as described above while the plate is moving,outputs I from the pixels of the line sensor including those receivinglight penetrating the air bubble will give a trace as shown in FIG. 5C.When the air bubble comes to the image of the boundary, output from theline sensor becomes lower than the level I₀ to give a trough which isfollowed by a peak higher than I₀. Outputs from the line sensor are fedto a computer where the outputs are processed to produce image data. Theimage data are then subjected to three value based contrast enhancementto enhance the contrast of an image represented by the image data, andthe thus contrast enhanced image is displayed on the screen. In thiscase, in correspondence with the entrapped air bubble, there appear onthe screen “dark spots” and “light spots” in this order, or a flawpattern of “dark and light” profile.

As described above, the order according to which “light spots” and “darkspots” appear on the screen, that is, flaw pattern of “light and dark”profile observed when a glass plate with a flaw is moved through theflaw detection system, varies depending on the type of the flaw, thatis, light deflecting property of the flaw. Therefore, one can determinethe type of a given flaw based on the flaw pattern it presents.Identification of the type of a given flaw based on the flaw pattern itpresents is hardly possible by using a conventional flaw detectionsystem dependent on a mesh structure comprising plural thin lightshielding septa and light transmitting slits as described in theJapanese Examined Patent Publication No. 3178644 as mentioned earlier in“Background Art” section, and is possible only when a flaw detectionsystem as described in this Description is employed which comprises asingle light transmitting portion, that is, a single slit instead ofplural slits which projects its linear image onto a linearly extendedimage pickup means in parallel with the long axis of the latter.Incidentally, the actually observed flaw pattern may include suchcomplicated patterns of light and dark spots which may extend indirections other than the direction of movement, that it cannot beascribed to any specific flaw pattern. Such a complicated flaw pattern,however, may be regarded as derived from a combination of flaws, afterit is analyzed into component patterns each ascribable to a differentspecific pattern.

An illustrative output is presented below when a glass plate entrappingan air bubble is submitted to the flaw detection system. FIG. 6 presentsan image displayed on the screen which is obtained by feeding signalsfrom the line sensor 41 via an image signal receiving unit to a computerwhere the signals are converted into image data which are thentransmitted to display without being subjected to any three value basedcontrast enhancement. The figure also shows vertical and horizontallight intensity profiles along a vertical cursor C1 and horizontalcursor C2. In the figure, the horizontal direction corresponds with thedirection of movement of the glass plate while the vertical directioncorresponds with the scanning direction of a sensor beam of the linesensor. The distribution of light intensities within the image is soobscure that it cannot be ascribed to any specific flaw pattern of“light spots” and “dark spots.”

According to this invention, the computer subjects the two-dimensionalimage data to three value based contrast enhancement to thereby enhancethe contrast of an image represented by the image data. The three valuebased contrast enhancement consists of firstly determining an averagelight intensity based on the two-dimensional image data, setting athreshold by a certain amount higher than the average or an over-averagethreshold, and another threshold by the same amount lower than theaverage or an under-average threshold using the average as a reference,taking one cluster of image data equal to or larger than theover-average threshold as representing “light spots,” another cluster ofimage data equal to or smaller than the under-average threshold asrepresenting “dark spots,” and the rest as representing “grey spots.” Inthe above description, the average is determined based on the entiretwo-dimensional image data. However, determining the average is notlimited to this method. The average may be determined dependent, forembodiment, on the image data derived from light passing through alimited two-dimensional space including a flaw.

A contrast enhanced image obtained by subjecting the aforementionedimage data to three value based contrast enhancement is shown in FIG. 7.The distribution of light intensities in the image is more enhanced. Itis possible to thereby make a flaw pattern comprising “light spots” and“dark spots” more distinct than a corresponding pattern obtained fromthe original image data.

From the figure it is obvious that, when an entrapped air bubble issubject to the flaw detection system, firstly “dark spots” appear whichare followed by “light spots”, that is, a flaw pattern of “dark andlight” profile appears with movement of the glass plate. In theparticular embodiment shown in FIG. 7, the “light spots” are followed byanother group of “dark spots.” This may be ascribed to a followingphenomenon: if the air bubble is large and optical defect due to thebubble is intense, part of light from the fluorescent lamp to penetratethe bubble is so much deflected that it falls out of the visual field ofthe line sensor. It is obvious from above that one can detect a smallair bubble entrapped in a glass plate based on a flaw pattern of “darkand light” profile, and a large air bubble based on a flaw pattern of“dark, light and dark” profile, with the maximum resolution of thepattern being equal to the resolution of the line sensor or to its n-thmultiple.

FIG. 8 shows an image obtained from image data prior to contrastenhancement when a glass plate entrapping a foreign object accompaniedwith a simple bump is applied to the flaw detection system. FIG. 9 showsa contrast enhanced image obtained from the same image data subjected tocontrast enhancement. From the figures it is obvious that, when the flawdetection system deals with an entrapped foreign object accompanied witha simple bump, firstly “light spots” appear which are followed by “darkspots”, that is, a flaw pattern of “light and dark” profile appears withmovement of the glass plate.

As described above, according to the flaw detection system as shown inFIG. 1, it is possible to distinguish at least between an entrapped airbubble and an entrapped foreign object based on the flaw patterns theypresent.

It will be readily obvious to those skilled in the art that, accordingto the flaw detection system shown in FIG. 1, when the moving directionof a test glass plate 3 with a flaw is reversed, the order in whichlight and dark spots appear with movement of the glass plate is reversedto the initially observed order.

Embodiment 2

The flaw detection system described above as Embodiment 1 incorporates alight shielding plate 22 consisting of a single plate. The flawdetection system of this embodiment incorporates instead a lightshielding plate including an additional plate. FIG. 10 shows the flawdetection system representing Embodiment 2. A second plate 23 added tothe light shielding plate is introduced so as to intercept scatteredlight which as a background light, out of light from a fluorescent lamp21, and to thereby reduce light diffusely reflected by a flaw. Theadditional light shielding plate 23 may be adjusted in its position inas much as it does not encroach on the visual field of the line sensor.

FIG. 11 is a diagram to show how the illuminating means looks like whenthe system of FIG. 10 is viewed by human naked eyes from its line sensor4. The two light shielding plates 22, 23 appear dark strips while thelighting surface of the fluorescent lamp 21 which is not masked by thetwo light shielding plates appears as a light strip 21. The cross-placedat the boundary between the light strip 21 and dark strip 22 marks thevisual center of the line sensor.

Here, it is appropriate to consider the case where this system isapplied for detecting a large air bubble entrapped in a glass plate byfollowing the pattern of light and dark spots generated by the bubblewith movement of the glass plate, with the maximum resolution of spotdetection being equal to the resolution of the line sensor or to itsn-th multiple. Then, a series of “dark spots,” “light spots” and “darkspots” or a flaw pattern of “dark, light and dark” profile as shown inFIG. 6 will be most frequently observed. FIGS. 12A, 12B and 12C, and 12Dshow illustrations corresponding with those of FIGS. 5A and 5B, and 5C,respectively. They illustrate how a flaw pattern of “dark, light anddark” profile is obtained from a large air bubble entrapped in a glassplate with movement of the glass plate. In FIG. 12A, the line sensorreceives light penetrating the front part of the air bubble entrapped inthe glass plate, and the intensity of light then received by the linesensor is lower than I₀ as indicated by a first trough in FIG. 12D. InFIG. 12B, the line sensor receives light penetrating a descendingshoulder of the air bubble, and the intensity of light then received bythe line sensor is slightly higher than I₀. In FIG. 12C, the line sensorreceives light penetrating the trail of the air bubble, and theintensity of light then received by the line sensor is again lower thanI₀.

Then, how the flaw detection system detects a foreign object fallingfrom above and adhering to the surface of a glass plate, or acrater-like print left by such a falling object will be described.

FIGS. 13A and 13B show the profiles of two flaws: one consists of afalling foreign object on the surface of a test glass plate accompaniedwith an annular ridge, and the other of a crater-like pit andsurrounding ridge left by such a falling foreign object. As shown inFIG. 13A, a flaw 63 resulting from a foreign projectile 62 striking thesurface of a glass plate is surrounded with an annular ridge, and imagedata obtained from this flaw, when subjected to contrast enhancement,gives a flaw pattern of “light and dark” profile similar to that of aflaw consisting of a bump. An exemplary flaw pattern of “light and dark”profile as described above is shown in FIG. 14A. A crater-like print 64left by a falling foreign object as shown in FIG. 13B gives, when theimage data is contrast-enhanced, a flaw pattern of “light, dark, lightand dark” profile as shown in FIG. 14B. It is found that a print left bya small foreign object gives a flaw pattern of “light and dark” profilewith the maximum resolution being equal to the resolution of the linesensor or its n-th multiple.

Next, with the flaw detection system as shown in FIG. 10, the linesensor is focused onto the surface of a glass plate, a dust on thesurface is traced, a contrast-enhanced image thereof is displayed on thescreen, and the profile of a flaw pattern obtained from the image isshown in FIG. 15. As is obvious from inspection of the figure, a dustonly generates “dark spots.” This is because a dust only intercepts theentry of light which reduces the intensity of light incident on the linesensor and causes dark spots to appear.

Accordingly with the flaw detection system where the line sensor isfocused onto the surface of a test glass plate, and introduction of anadditional light shielding plate 23 restricts the entry of scatteredlight, it is possible to distinguish between a flaw accompanied with anoptical defect and a simple blemish such as a dust devoid of opticaldefect by analyzing respective sequential patterns of “light spots” and“dark spots” observed in their contrast-enhanced images.

It will be readily obvious to those skilled in the art that, accordingto the flaw detection system shown in FIG. 10, when the moving directionof a test glass plate 3 with a flaw is reversed, the order in whichlight and dark spots appear with movement of the glass plate is reversedto the initially observed order.

The dust includes debris which is generated while a master glass plateis cut into elementary glass plates and which may adhere to the surfaceof elementary glass plates as a foreign object. Since such debris, whenreceiving light, deflects it so abnormally that it is possible to detecta flaw caused by it by analyzing the optical defect associated with itlike other flaws accompanied with an optical defect. However, debrisgenerated as a result of cutting a master glass plate may often cause acomplicated flaw pattern comprising “light spots” and/or “dark spots.”

With Embodiments 1 and 2, an image of the boundary between the lightingportion and darkening portion of the illuminating means 2 runs in adirection perpendicular to the direction in which a test transparentplate is moving. However, the angle between the two directions may notbe an exact rectangle but close to a rectangle. If it is desired toparticularly detect a flaw which has a direction corresponding to thedirection in which a glass melt is allowed to flow prior to theformation of a plate, the angle between the moving direction of theglass plate and the direction of the visual field of the line sensor maybe adjusted as appropriate so that the flaw detection system can bestdetect such a flaw.

Variant Embodiments

Although the flaw detection system of Embodiment 1 comprises anilluminating means which consists of a fluorescent lamp and a lightshielding plate, the system configuration is not limited to this. Forembodiment, the single fluorescent lamp may be substituted for twofluorescent lamps 21 as shown in FIG. 16.

Alternatively, the illuminating means may comprise a plurality offluorescent lamps 21 and a light shielding plate 24 and light scatteringplate 25 placed above the lamps as shown in FIG. 17.

With the embodiments and variant embodiments mentioned above, the lightsource consists of one or plural fluorescent lamp(s). However, the lightsource may include, in addition to a fluorescent lamp(s), forembodiment, a halogen lamp(s). Namely, light guided by plural opticalfibers from a halogen lamp(s) may be used as a light source.Furthermore, the light source may include a rod-like LED.

The transparent plate to which the flaw detection system of theinvention is applied is not limited to those in which the surfaces areuniformly flat and run in parallel. The plate may include those thathave a gentle curvature like a display panel. The plate may furtherinclude plate strips having a moderate length, or long continuous platestrips. The flaw detection system of the invention is also applicable tosemi-transparent plates, as long as they are capable of transmittinglight.

INDUSTRIAL APPLICABILITY

As detailed above, according to the testing method and system of theinvention, it is possible to detect a flaw accompanied with an opticaldefect if any on or in a glass plate, by analyzing a pattern observed inits contrast-enhanced image comprising “light spots” and “dark spots.”Particularly, it is possible to determine the type of the flaw byfollowing the order in which “light spots” and “dark spots” appear withmovement of the glass plate, and how “light spots” and “dark spots”combine.

According to the testing method and system of the invention, it is alsopossible to detect a flaw not accompanied with any optical defect suchas a simple blemish like a dust adherent on the surface of a glass platewhich can be removed. Thus, it is possible according to the testingmethod and system of the invention to distinguish a flaw consisting of asimple blemish from a flaw consisting of a bump or notch formed on thesurface, or of a foreign object or air bubble entrapped in the glassplate, and to separately treat glass plates having the former flaw fromthose having the latter flaw. By virtue of this, the flaw detectionmethod and system of the invention does not necessarily require, for thetest, a clean space as provided by a clean booth or the like, which willhelp to reduce costs. Moreover, according to the method and system ofthe invention, it is also possible to acquire information about whatkind of handling causes frequent adherence of dusts on the surface ofglass plates. This information enables one to improve the polishingprocess in the manufacture of glass plates.

1. A flaw detection system for detecting a flaw accompanied with an optical defect if any on or in a sheet-like transparent body which is moved, comprising: a linearly extended (one-dimensional) image pickup means placed on one side of the sheet-like transparent body: an illuminating means placed on the opposite side of the sheet-like transparent body, the illuminating means comprising a lighting portion and a darkening portion, the lighting and darkening portions of the illuminating means being constituted respectively with a light source, a first light shielding plate which is placed between the light source and sheet-like transparent body to shield part of the light source such that an image of the edge of the first light shielding plate forms a visual field of the image pickup means, and a second light shielding plate is placed between the illuminating means and sheet-like transparent body so as to intercept scattered light which is a background light, out of light from the light source to thereby reduce light diffusely reflected by a flaw; and an image processing means in which two-dimensional image data are created from one-dimensional output from the image pickup means, the image data are subjected to three value based contrast enhancement to convert said data into contrast enhanced image data which are then utilized to display a contrast enhanced image, the sequential pattern (flaw pattern) of light spots and dark spots is traced which appears in the contrast enhanced image with movement of the sheet-like transparent body.
 2. A flaw detection system according to claim 1, wherein identification of the type of a flaw is achieved by determining the sequential order in which light spots and dark spots constituting the flaw pattern appear with movement of a sheet-like transparent body or the combination of light spots and dark spots in the flaw pattern.
 3. A flaw detection system according to claim 2, wherein the types of flaws to be distinctly identified comprise at least a flaw consisting of a foreign object and a flaw consisting of an air bubble.
 4. A flaw detection method according to claim 2, wherein the types of flaws to be distinctly identified comprise at least one chosen from the group comprising flaws consisting of a foreign object, air bubble, bump, notch, falling foreign object, and print left by a falling foreign object.
 5. A flaw detection system according to claim 1, wherein the linearly extended (one-dimensional) image pickup means brings its focus close to a sheet-like transparent body; and when a “dark” pattern appears in a contrast enhanced image, the system determines that the flaw responsible for the “dark” pattern is a dust adherent on the surface of the sheet-like transparent body.
 6. A flaw detection system according to claim 1, wherein the sheet-like transparent body comprises a glass plate.
 7. A flaw detections system according to claim 1, wherein the sheet-like transparent body comprises a glass plate whose surface is coated with a transparent film.
 8. A flaw detection system according to claim 7, wherein the type of flaw to be identified further comprises a pinhole if any present in the transparent film. 